In general, ISI (Inter-Symbol Interference) according to multiple paths is generated to receive signals when transmitting signals through a multi-path channel. In particular, the ISI is further reinforced in the case of high-speed data transmission since a symbol period is less than a delay spread of a channel, and hence, more complex receiving methods are required so as to compensate for distortion caused by the ISI and restore accurate transmit signals. In order to reduce a distortion phenomenon of signals caused by the ISI, the symbol period is to be greater than the delay spread of the channel, and the OFDM method has been proposed as a modulation method for simply compensating for the distortion of the multi-path channel.
The OFDM method uses a plurality of subcarriers having orthogonality to transmit data, differing from the transmission method which uses single carriers. That is, the OFDM method performs serial and parallel conversion on the input data by the number of subcarriers used for modulation, and modulates the respective converted data by using the corresponding subcarriers to thereby maintain data rates and increase the symbol period for each subcarrier by the number of subcarriers. Since the OFDM method uses subcarriers having orthogonality, it has better bandwidth efficiencies and longer symbol periods compared to the existing FDM (Frequency Division Multiplex) method, and it has an ISI-resistance characteristic compared to the single carrier modulation method.
The modulation and demodulation process at a transmitter and a receiver in the OFDM system corresponds to execution of IDFT (Inverse Discrete Fourier Transform) and DFT (Discrete Fourier Transform), and it can also be realized effectively by using IFFT (Inverse Fast Fourier Transform) and FFT (Fast Fourier Transform). Further, when a guard interval which is longer than the delay spread of the channel is inserted for each symbol period, the inter-carrier orthogonality is maintained to generate no ICI (Inter-Carrier Interference), and no OFDM symbols caused by the multi-path channel are superimposed to completely eliminate adjacent inter-symbol ISI.
The conventional TDMA-based cellular system uses a different frequency in the adjacent cell to reduce interference by other cells. The method, however, reduces the total system capacity because of low frequency reuse efficiencies. The existing CDMA-based cellular system, such as the IS-95, the CDMA2000, and the W-CDMA, uses spread spectrum codes to decrease interference caused by channels of other cells by the spread components, and averages the interference of various channels of the cells to use the identical frequency in the adjacent cell. However, the IEEE802.11a, the Hiperlan/2, and the DAB (Digital Audio Broadcasting) on the conventional OFDM basis have been designed in environments other than that for the cellular system, and no consideration for reducing the inter-cell interference has been studied. In this instance, the inter-cell interference is reduced by using a different frequency in the adjacent cell, but the frequency reuse efficiencies are degraded, and the system capacity is decreased.
In order to improve the frequency reuse efficiencies in the above-noted OFDMA-based cellular system, the dynamic packet allocation method and the dynamic channel allocation method are used (refer to ‘Network-assisted resource management for wireless data networks,’ by X. Qiu, K. Chawla, J. C. I. Chuang, and N. Sollenberger, IEEE Selected Areas Commun, Vol. 19, pp. 1222-1234, July 2001). Adjacent cells communicate with signaling, and they do not transmit data to the channel which another adjacent cell uses. This method gives good theoretical performance, but it is not appropriate for the actual systems because of a large amount of overheads.
Further, the OFDMA-based cellular system uses the frequency hopping method to average the inter-cell interference, and uses different frequency hopping patterns for the respective cells so that a channel of a cell may uniformly collide with many channels of another cell, and the inter-cell interference may accordingly be averaged, which is disclosed by the OFDMA method of the IEEE802.16a. However, since the IEEE802.16a averages the interference without distinguishing the channels, the required SNR (Signal to Noise Ratio) is also averaged since the channels are mixed, and hence, it is difficult to control the inter-cell interference.